Hyperpolarized [¹³C]ketobutyrate, a molecular analog of pyruvate with modified specificity for LDH isoforms

Purpose The purpose of this study was to investigate ¹³C hyperpolarization of α-ketobutyrate (αKB), an endogenous molecular analog of pyruvate, and its in vivo enzymatic conversion via lactate dehydrogenase (LDH) using localized MR spectroscopy. Methods Hyperpolarized (HP) ¹³C MR experiments were conducted using [¹³C]αKB with rats in vivo and with isolated LDH enzyme in vitro, along with comparative experiments using [¹³C]pyruvate. Based on differences in the kinetics of its reaction with individual LDH isoforms, HP [¹³C]αKB was investigated as a novel MR probe, with added specificity for activity of LDHB-expressed H ("heart"-type) subunits of LDH (e.g., constituents of LDH-1 isoform). Results Comparable T₁ and polarization values to pyruvate were attained (T₁ = 52 s at 3 tesla [T], polarization = 10%, at C₁). MR experiments showed rapid enzymatic conversion with substantially increased specificity. Formation of product HP [¹³C]α-hydroxybutyrate (αHB) from αKB in vivo was increased 2.7-fold in cardiac slabs relative to liver and kidney slabs. In vitro studies resulted in 5.0-fold higher product production from αKB with bovine heart LDH-1, as compared with pyruvate. Conclusions HP [¹³C]αKB may be a useful MR probe of cardiac metabolism and other applications where the role of H subunits of LDH is significant (e.g., renal cortex and brain).